This invention relates to the purification of the exhaust gas of an engine or combustion device, and in particular to purification of NOx in exhaust gas with excess oxygen.
In a diesel engine or gasoline engine which performs lean burn, good fuel-cost performance is obtained, but it is difficult to purify the nitrogen oxide (NOx) in the exhaust gas as the exhaust gas contains excess oxygen.
In this regard, U.S. Pat. No. 5,473,887 and Tokkai Hei 6-212961 published by the Japanese Patent Office in 1994 disclose a NOx absorber which is disposed in the exhaust gas passage of the engine.
This NOx absorbing material comprises platinum (Pt) and barium (Ba) which are supported on a catalyst substrate.
The NOx absorbing material absorbs NOx when the oxygen concentration of the exhaust gas of the exhaust gas passage is high, and it discharges NOx as the oxygen concentration decreases.
Even in lean exhaust gas, the NOx absorbing material discharges NOx if the degree of leanness falls.
Here, the state where the oxygen concentration of the exhaust gas is higher than the oxygen concentration corresponding to combustion gas from a fuel-air mixture having the stoichiometric air-fuel ratio is referred to as lean, and the state where the oxygen concentration of the exhaust gas is lower than this is referred to as rich.
The discharged NOx is reduced using a catalyst. However, for this to occur, the oxygen concentration of the exhaust gas must have a value equivalent to the stoichiometric air-fuel ratio, or rich.
Therefore, when the NOx absorption amount of the NOx absorbing material approaches a saturation amount, running at a lean air-fuel ratio must be temporarily interrupted, and the engine must be run at the stoichiometric air-fuel ratio or a rich air-fuel ratio so that the NOx absorbed by the NOx absorbing material can be discharged and reduced. However, if the engine is periodically run at the stoichiometric air-fuel ratio or a rich air-fuel ratio in this way, the advantage of low fuel consumption of a diesel engine or a lean burn gasoline engine is partly lost.
The aforementioned Tokkai Hei 6-212961 also discloses a diesel engine wherein a NOx absorbing material and a hydrocarbon (HC) absorbing material are provided in the exhaust passage.
This diesel engine performs a supplementary fuel injection in the explosion process as NOx is discharged from the NOx absorbing material, and when the exhaust gas temperature rises due to this supplementary injection, the HC absorbing material discharges HC. By using this HC for the reduction of NOx which is discharged from the NOx absorbing material, the NOx reduction efficiency is increased.
However, the HC amount discharged from a diesel engine is less than the NOx discharge amount. Therefore, it is difficult to make the HC absorbing material absorb an HC amount corresponding to the NOx adsorption amount of the NOx absorbing material. If a supplementary injection is performed in order to provide the necessary HC amount, the low fuel consumption advantage of a diesel engine is again partly lost.
It is therefore an object of this invention to efficiently discharge NOx from an NOx absorbing material and efficiently reduce the discharged NOx in an atmosphere wherein excess oxygen is present.
In order to achieve the above objects, this invention provides a device for purifying exhaust gas containing nitrogen oxides in an exhaust passage of a combustion device. The exhaust gas purifying comprises a nitrogen oxide absorbing material which absorbs nitrogen oxides when a reduction component concentration of the exhaust gas is lower than a predetermined concentration, and discharges nitrogen oxides when the reduction component concentration of the exhaust gas is higher than the predetermined concentration, a catalyst which promotes reduction of discharged nitrogen oxides by a reduction component in the exhaust gas, a mechanism for decreasing the reduction component concentration of the exhaust gas, a mechanism for increasing the reduction component concentration of the exhaust gas, and a microprocessor. The microprocessor is programmed to control the decreasing means so that the reduction component concentration of the exhaust gas is less than a first predetermined concentration for a first predetermined time period, and control the increasing means so that the reduction component concentration of the exhaust gas is higher than a second predetermined concentration for a second predetermined time period after the first predetermined time period has elapsed.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.